A threesome as a one-off (so to speak). A three-in-a-bed is a sexual encounter between three people, usually (at least for two of those involved) for the first and last time.

A phrase beloved of the British tabloid press is "three-in-a-bed romp", which stresses bother the impermanence and the (just-for-)fun nature of the experience, thus engaging the sexual interest and desire, the envy and thus the moralising indignation of the newspaper's readers all at once!